1. Prior Art Solutions
There are several techniques known to date for the automatic modeling of vast urban environments which all aim at the same goal of reducing the cost of modeling towns and cities. These techniques rely for example on methods of photogrammetry, radar image analysis, and the use of elevation maps obtained from airborne scanners.
Most of these techniques provide what are called “2D½” models consisting of a set of footprints of buildings which are associated with the altitude at the base of the buildings and their eaves-gutter height. This modeling is therefore prismatic since, in order to obtain a 3D depiction of a building, it consists of the extrusion of the building footprint along its eaves-gutter height. Once the walls have been rebuilt by means of this extrusion technique, then in order to obtain a realistic rendering of the city or of the building, it is also necessary rebuild a 3D representation of the roof.
To date, there are several known techniques of transmission of a roof model enabling a remote browser to carry out a rebuilding such as this.
Thus, a first technique relying on the VRML (Virtual Reality Modeling Language) consists of the transmission of the roof model in the form of a set of polygons corresponding to different faces, also called sloping sides, of the roof to be rebuilt (known as the “indexed face set” function in VRML).
Another technique proposed by P. Felkel and S. Obdrzalek in “Straight skeleton implementation” (Spring Conference on Computer Graphics, 1998), consists of the automatic reconstruction of a roof from the footprint of the building, in determining its straight skeleton, and in carrying out an elevation of its peaks.
2. Drawbacks of the Prior Art
These different techniques however cannot be used to comply with the following constraints jointly:                low bit rate of most of the current communications networks, and        need for realism of viewing by the final users.        
Thus, the technique which consists of the transmission of a roof model, in VRML, in the form of a set of its faces generates a very large volume or even a prohibitively large volume of data in the case of complex roofs. While, at reception, it enables the rebuilding of most shapes of roofs in a fairly realistic way, it is quite unsuited in the case of a town model which may contain several hundreds of thousands of buildings and for which the transmission time, through the network, of the set of 3D roof models will be considerable.
The technique of automatic roof building proposed by P. Felkel and S. Obdrzalek for its part is less costly in terms of resources since, for providing the roof structure, it is based on the straight skeleton of the footprint of the building which furthermore has already been transmitted to the user. However, this technique does not enable the rebuilding of 3D depictions of hip type roofs, also called roofs with several sloping sides, wherein the sloping sides have the same slope angle relative to the horizontal. This technique is therefore not suited to the depiction of models of real towns in which the buildings may have roofs of all types and especially complex gable roofs and hip roofs having a multiplicity of sloping sides.